This invention relates to the use of two or more different metal compounds, preferably in the same reactor, used to produce polyolefins.
Metallocene compounds are of particular interest in the polyolefin industry today for their use as polymerization catalysts. For example both biscyclopentadienyl and monocyclopentadienyl transition metal compounds (particularly of groups 4, 5 and 6) are known to polymerize olefins when used in combination with an activator, such as an alumoxane or a non-coordinating anion. Likewise U.S. Pat. No. 5,527,752 discloses a new class of olefin polymerization catalysts based on complexes of a transition metal having pi bonded ligands and heteroallyl moieties combined with an activator such as an alumoxane or a non-coordinating anion. Further, copending U.S. patent application 09/103,620 filed Jun. 23, 1998 (published as WO 99/01460) assigned to Union Carbide discloses new transition metal compounds based on bidentate ligands containing pyridine or quinoline moieties for use on olefin polymerizations.
These new catalysts, such as the metallocene polymerization catalysts (i.e. those containing a transition metal bound to at least one cyclopentadienyl, indenyl or fluorenyl group), have recently been used to produce new resins having desirable product properties. For example metallocene catalyst systems are used by Exxon Chemical Company to produce EXCEED(trademark) type polyethylene resins. While these resins have excellent toughness properties, particularly dart impact properties, they, like other metallocene catalyzed polyethylenes can be difficult to process on older equipment. One of the means used to improve the processing of metallocene catalyzed polyethylenes is to blend them with high density polyethylene. This, however, is expensive and adds a cumbersome blending step to the manufacturing/fabrication process.
Higher molecular weight confers desirable mechanical properties and stable bubble formation onto polyethylene polymers. However, it also inhibits extrusion processing by increasing backpressure in extruders, promoting melt fracture defects in the inflating bubble and potentially, promotes too high a degree of orientation in the finished film. To remedy this, one may form a secondary, minor component of lower molecular weight polymer to reduce extruder backpressure and inhibit melt fracture. Several industrial processes operate on this principle; using multiple reactor technology.
Another option used to address this processability problem has been to try to produce the two polymers together at the same time in the same reactor using two different catalysts. Mobil in their patent application WO 99/03899, discloses using a metallocene type catalyst and a Ziegler-Natta type catalyst in the same reactor to produce a bimodal molecular weight distribution (Mw/Mn) high density polyethylene. These resins however still do not have a preferred balance of processability and strength properties.
Other dual catalyst systems have been used in the past for a variety of reasons. For example WO 98/02247 discloses a dual catalyst system of a metallocene and a non-metallocene (TiCl4+alcohol) treated with the contact product of dialkylmagnesium and trialkylsilanol. WO 98/02247 discloses dual metallocene systems and describes the idea that the two different transition metal sources exhibit a different hydrogen response under the same polymerization and hydrogen conditions as critical. Hydrogen response is the sensitivity of the catalyst to manipulation by adding or subtracting hydrogen to or from the polymerization system to produce different products. Likewise, U.S. Pat. No. 4,935,474 discloses olefin polymerization in the presence of two or more metallocenes (activated with alumoxane) each having a different propagation and termination rate constants. U.S. Pat. No. 5,464,905 discloses a molding polymer composition which comprises a copolymer blend produced from a copolymer produced from two different metallocenes combined with alumoxane and a second copolymer produced with a metallocene and alumoxane. Liquid mixtures of many classes of catalysts are disclosed for use in gas phase polymerization in U.S. Pat. No. 5,693,727. U.S. Pat. No. ""727 discloses that more than one liquid metallocene may be employed. Similarly, EP 0 770 629 A discloses a process to produce bimodal polymers using two reactors in series. In some circumstances only the reaction conditions and monomer feeds are changed in the second reactor. In other circumstances a second different catalyst is added to the second reactor.
Mitsui, for example, and others produce a processable bimodal molecular weight distribution (MWD) high density polyethylene product under the Tradename HIZEX(trademark) which is considered a worldwide standard for this type of HDPE product. HIZEX(trademark) is produced in two or more reactors at a substantial cost. While bimodal MWD HDPE products have these desirable characteristics, they can be inherently costly to produce because they require a series- or cascade-reactor system. In such systems, each reactor in a multiple reactor process produces a single component of the final product. Thus, there is a need in the art to produce a processable polyethylene having a good balance of haze and gloss with improved physical properties in one reactor.
An option used to address this processability problem has been to try to produce two polymers together at the same time in the same reactor using two different catalysts. Mobil in PCT patent application WO 99/03899, discloses using a metallocene type catalyst and a Ziegler-Natta type catalyst in the same reactor to produce a bimodal molecular weight distribution (MWD) high-density polyethylene (HDPE). These resins however still do not have a preferred balance of processability and strength properties. Thus, there is a desire for a processable polyethylene polymers arising from a single reactor process having desirable processing, mechanical and optical properties.
This invention provides a dual catalyst system that can be used in one reactor to produce processable polyethylene.
This invention relates to a process to polymerize olefins comprising reacting one or more olefins with a catalyst system comprising at least two metal compounds and an activator in a gas or slurry phase reactor. The first metal compound is preferably a metallocene and the second metal compound is preferably a transition metal compound based on bidentate ligands containing heterocycle moieties. Preferably the metal compounds are selected in such a way that one produces high molecular weight polymer and another produces lower molecular weight polymer.